Conversion of tonal character of aural signals



CONVERSION OF TONAL CHARACTER 0F AURAL SIGNALS Filed Oct. '2, 1965 D. J.CAMPBELL Nov. 4, 1969 5 Sheets-Sheet l M 2 IL 3 (V I T L I F 1 o I T 2 AAh h 4 88 mm m Im L m 4 H A S T. I I I N E M L G C P I 0 U N M S R A P u5 m l K n u P m 1 U! W m H M/VE/VTOR DONALD J. CAMPBELL '8'A a'n IG'CISIGNAL.

PROCESSING UNIT PICKUP ATTORNEYS 1969 D. J. CAMPBELL CONVERSION OF TONALCHARACTER OF AURAL SIGNALS Filed Oct. 7, 1965 5 Sheets-Sheet 2 INVENTORDONALD J. CAMPBELL A TTORIVYS Nov. 4, 1969 D. J. CAMPBELL 3,476,863

couvmns on OF TONAL CHARACTER OF AURAL SIGNALS Filed Oct. 7, 1965 5Sheets-Sheet 3 536 527 m m I529 50l FIG. 3

5m, INVENTOR PW, Md? //V 4 ATTORNEYS Nov. 4, 1969 D, J. CAMPBEL3,476,863

CONVERSION OF TONAL CHARACTER OF AURA L SIGNALS Filed Oct. '7, 1965 5Sheets-Sheet 4 70; N 4 MVEWOR DONALD J. CAMPBELL Nov. 4, 1969 D. J.CAMPBELL 3,476,863

v CONVERSION OF TONAL CHARACTER 0F AURAL SIGNALS Filed Oct. 7. 1965 5Sheets-Sheet 5 INVENTOR DONALD J. CAMPBELL FWJW 4w ATTORNEYS UnitedStates Patent O 3,476,863 CONVERSION OF TONAL CHARACTER OF AURAL SIGNALSDonald J. Campbell, Cincinnati, Ohio, assignor to Chicago MusicalInstrument Co., Lincolnwood, 11]., a corporation of Delaware Filed Oct.7, 1965, Ser. No. 493,773 Int. Cl. G10h 1/00; H03k 5 /01 US. Cl. 841.0130 Claims ABSTRACT OF THE DISCLOSURE The present invention relates tothe art of electronic music generation. More particularly, thisinvention is concerned with the conversion of an aural input of one s ttone color or timbre to an aural output having a different tone color ortimbre, and/ or a different pitch, or ampitude, or othercharacteristics. As used in the present specification and claims, unlessotherwise indicated expressly or by context, the term aural input isintended to include not only the compressional sound wave per se, butalso a corresponding electrical signal that may be generatedconcurrently with a sound wave, as for example by the use of anelectromagnetic or other direct pick" up as is employed with electricguitars.

In accordance with the general aspectsof the present invention, an auralinput signal is analyzed to obtain its fundamental frequency, and eachcycle is analyzed to obtain its amplitude. Output waves of highharmonics content are then produced, for example square and sawtoothwaves, which correspond in frequency to the fundamental of the auralinput signal, and correspond cycle by cycle to the amplitude of theaural input signal. The resultant signals are then processed throughwell known tone color filters to obtain desired variations in the timbreof the signal, and thence arereconverted into an aural signal of desiredtone color; different from that of the aural input. Further, as willbereadily apparent, the processing of the signal can include frequencydoubling and dividing in order to change the pitch of the signal asdesired.

The processing of an aural input signal as above described, may be usedfor example to convert a voice input to an instrumental output, convertan input from one instrument to an output having the tone color of adifferent instrument, and converting an input signal of one tone colorand fundamental frequency to an output of a difierent tone color andsame or different fundamental frequency, or same tone color anddifferent fundamental frequency. 7

It is therefore among the objects of the present invention to provide:for the electronic processing'of aural input signals to aural outputsignals; for the conversion of an aural input signal to waves of highharmonic content having the same fundamental frequency as the input, ormultiples thereof, and corresponding in amplitude to the input signal,cycle by cycle; for the conversion of an aural input signal of one tonecolor to an aural output signal of another tone color; and for theconversion of an aural input signal of one frequency to an aural output3,476,863 Patented Nov. 4, 1969 ice signal of a multiple or fraction ofthe input frequency.

Other objects and advantages of the present invention will becomeapparent to those skilled in the art from a consideration of thefollowing detailed description of one specific embodiment of theinvention had in conjunction with the accompanying drawings, in whichlike numerals refer to like or corresponding parts, and wherein:

FIG. 1 is a block diagram of an overall system from aural input to auraloutput;

FIG. 2 is a schematic circuit diagram of a signal processing unitembodying the present invention; and

FIGS. 3, 4, 5, and 6 are waveform charts showing the various waveformsappearing in the processing unit of FIG. 2 in their relative timerelationships.

Referring first to the block diagram of FIG. 1, it will be seen that apickup unit 10, which may be a microphone for transducing a voice orinstrumental sound to an electrical signal, transmits the input signalto the processing unit 11, where the signal is transposed into squareand sawtooth waves related in frequency and amplitude to the inputsignal. The square and sawtooth wave outputs indicated as 4 foot, 8 footand 16 foot outputs, are carried by busses 12 to appropriate tone colorfilters 13. The outputs of the filters 13 are combined in amplifier 14,and thence transmitted to output speaker 15. Obviously, any number ofpickups 10 may be provided, depending upon the number of inputsutilized, as indicated in FIG. 1 by pickups and processing unitsnumbered I through 11. Each processing unit may of course be identical,and consequently the detailed description of one such unit will besufiicient.

Processing unit 11 is shown in the schematic circuit diagram of FIG. 2.Many of the waveforms present in the operation of the circuit of FIG. 2have been indicated thereon to facilitate an understanding of thiscircuit, and they have been numbered in accordance with the waveforms inthe chart of FIG. 3.

The aural input signal represented by a waveform 501 is coupled from thepickup microphone 10 through capacitor 21 to the base of clippingamplifier 22. The clipped and inverted waveform 502 constitutes theoutput of amplifier 22, and it contains only frequency information ofinput waveform 501. Waveform 502 is then differentiated by capacitor 23and resistor 24 into wayeform 503, then rectified by diode 25 intowaveform 504. It 'will be observed that at this point in processing theinputsignal 501, a trigger is provided for each negative going zerocrossing of waveform 501. The positive going crossings have beeneliminated by the diode 25.

Waveform 504 is converted to a sawtooth Waveform 505 by the interactionof the transistors 26 and 30, and the network of resistors 28 and 29 andcapacitors 27 and 32. Between trigger pulses of waveform 504, transister26 is biased below cutoff, thereby causing capacitor-27 to be slowlycharged through resistors 28 and 29. Each trigger pulse of waveform 504activates transistor 2i6'and causes capacitor 27 to be rapidlydischarged thereth-rough, thus producing the sawtooth waveform 505. Theoutput of the sawtooth generator is converted to a lowimpedance throughemitter follower transistor 30 and anti-oscillation resistor 31.Bootstrap feedback capacit'or' 32 couples the output of emitter follower30 to a point between resistor 28 and 29, to increase the linearity ofthe waveform 505.

The output 505 of emitter follower 30 is converted into a square wave,i.e. a rectangularwave symmetrical in time, by means of a squarercircuit comprising transister 33 and diode 34. When the linear sawtoothwave 505 is coupled through capacitor 35 into a load, the new D.C. levelcreated depends on the nature of the load. If the load'is linea-r andbilateral, the new level is 524 indicated in FIG. 3, area 520 beingequal to area 521, and time 522 being equal to time 523. A bilateralload is provided by diode 34 conducting during time 522 and transistor33 conducting during time 523. A linear load is insured by providingmost of the load impedance in resistor 36. These provisions result inlevel 524 being held at the conduction point of the base of transistor33. Thus transistor 33 conducts during time 523, is cut off during time522, and square wave 506 is thereby produced at the output of transistor33. Feedback network 37 and 38 provides for transistor interchange andcomponent tolerances. It will be observed that the symmetry of squarewave 506 on the time axis is substantially independent of frequency.Also it should be noted that this symmetry of 506 is obtained despiteany asymmetry that may exist in input waveform 501. This result is hadby utilizing only the negative going zero crossings of input waveform501 as obtained at 504, establishing frequency on that basis as insawtooth waveform 505, and then dividing the frequency period into twoequal halves, as in square waveform 506.

Having established a symmetrical waveform 506 equal in frequency to thefundamental frequency of the input signal, there remains to beestablished for each cycle of this waveform an amplitude correspondingto that of the corresponding cycle of the input signal. When controllingthe amplitude of a square wave, it is not necessary to establishamplitude until one-half of a given cycle has occurred. This fact makesit possible to measure the peak of a controlling wave during each firsthalf of the square wave cycle, to store this amplitude information, andthen to use it to control the amplitude of each second half of thesquare wave. By clearing the storage element at the end of each cycle,the amplitude of each square wave cycle then becomes a function of thepeak amplitude of the corresponding cycle of the controlling wave.

This function is accomplished by the amplitude control circuitcomprising coupling capacitor 39, diode 40, storage capacitor 41,emitter follower transistor 42, resistor 43, amplifier '44, diode 45a,differentiating network of capacitor 45 and resistor 46, and transistor47. The input waveform 501 is coupled through capacitor 39 to diode 40,which passes only the positive going portions of Waveform 501, therebytrapping the positive peak voltage of 501 on storage capacitor 41,indicated as waveform 509. The emitter follower 42 passes this waveformthrough resistor 43 to point 48 where it is combined with waveform 506coupled through diode 45a. Positive portions of waveform 506 are notpassed by diode 45 to point 48, hence during that portion of the cyclethe positive plateau of waveform 509 controls the voltage at point 48.The negative portions of waveform 506 are passed by diode 45 to point48, causing the voltage of waveform 509 to be dropped across resistor43, and bringing the voltage at point 48 below cut off value for thebase of transistor 44. Accordingly, waveform 510 is generated at point48, whose amplitude for each cycle is a direct function of the peakamplitude of waveform 501 for that cycle.

Considering the amplitude control circuit with more detailed referenceto the waveforms in FIG. 3, it will be seen that waveform 501 chargescapacitor 41 during the time 523, resulting in the 526 portion ofwaveform 509. Waveform portion 526 has substantially the same shape andamplitude as the portion 527 of waveform 501, hence amplitude 530 ofwaveform 509 is substantially equal to amplitude 529 of waveform 501.During time 523 square wave 506 is conducted to point 48 by diode 45alowering the base of transistor 44 below cut off. During this timevoltage of waveform 509 has no effect on the transistor 44 and isdropped across resistor 43. Since diode 45a is cut off during the time531, the voltage of waveform 509 controls, establishing the amplitude ofwaveform 510 at the value 530.

Waveform 510 applied to the base of amplifier 44 produces the desiredsquare wave output signal embodied in waveform 511. Waveform 511 isdifferentiated by capacitor 45 and resistor 46 into waveform 512, andeach positive going portion of waveform 511 produces a correspondingpositive spike in waveform 512 triggering transistor 47 into momentaryconduction to discharge storage capacitor 41.

The foregoing description relates to the basic flow of a signal beingprocessed in unit 11. There remain several refinements to the processingunit that will now be de scribed. It will be recalled that generation ofthe basic signal of the procesing unit, i.e., waveform 506, depends uponnegative going zero crossings of input signal 501, producing one squarewave cycle for each such zero crossing. These negative going zerocrossings are registered in the spikes of waveform 504. As illustratedin the drawings, input signal 501 has one negative going zero crossingper fundamental cycle. However, this input is usually a complex signalas illustrated by the secondary minimum at 536 in waveform 501 (see FIG.3). As the amplitude of this input signal decreases, changes in thewaveform may occur whereby a point Will be reached where the minimumpoint 536 crosses the Zero value, and there would thereby be produced afalse or extraneous zero crossing which would immediately result in adoubling of the frequency of waveform 506. To eliminate this occurrence,a blanking signal is generated to blank out waveform 504 for most of theperiod between spikes, when such extraneous zero crossings are likely tooccur. The blanking circuit is embodied in the feedback path betweencapacitor 51 and transistor 52.

Sawtooth waveform 505 is coupled through capacitor 51 to the base oftransistor 50. Transistor 50 and its associated circuitry responds tothe sawtooth input in the same manner as squarer 33 to produce a waveoutput, except the load is designed to produce the rectangular waveform507 unsymmetrical in time, instead of the symmetrical waveform 506, withportion 532 extending for about of the time period 525. Waveform 507,after passing through a resistance-capacitance-diode network appears aswaveform 508 at the base of transistor 52. The voltage level of portion533 of waveform 508 is chosen to render transistor 52 conductive, andnon-conductive or cut off at a value slightly below. As a result, thebase of transistor 26 is blanked during the period 533 when transistor52 is conductive. The shape and values of waveform 508 are selected toblank out the base of transistor 26 from approximately the 5% point toapproximately the 90% point of the period 525. The margins at either endof the blanking period are necessary to assure that the fundamentalsignal frequency zero crossings are not also blanked out. Otherwise, thebase of transistor 26 would be driven positive by the true signal zerocrossing spikes of waveform 504 and simultaneously negative bytransistor 52, and the system would not operate.

Network 54, 55, 56 and 57 eliminates transients 1010 and 1011 shown inthe waveform diagram of FIG. 4. FIGURE 4'shows the operation of theblanking circuit under transient conditions, whereas, the abovedescription and the waveform diagram of FIG. 3 shows its operation understeady state conditions. Waveform 1001 corresponds to sawtooth waveform505 appearing at the output of transistor 30 when an input signal beginsat instant 1007 and ends at instant 1008. Waveform 1002 appears at thebase of transistor 50, and waveform 1003 corresponds to waveform 507appearing at the collector of transistor 50. Differentiating network 54,55 produces waveform 1004, and diode 56 passes ony the negative portionthereof, producing waveform 1005 and eliminating transients 1010 and1011. These transients can cause frequency halving when notes arerapidly repeated; because if a second note begins during time 1009, itis possible for a true or desired zero crossing to occur during the timewaveform 504 would be blanked out by transient 1010, and if just onedesired or true zero crossing is blanked out the entire system mustoperate at half frequency thereafter for the remainder of the note,rejecting each alternate true zero crossing as an extraneous zerocrossing. r l Y Waveform 1005 has a positive quiescent value which ifcoupled to transistor 52 would blank out the base of transistor 26between notes and render the system unresponsive to the beginning of anynote. However, passing Waveform 1005 through capacitor 59 results inwaveform 1006, corresponding to waveform 508, having a quiescent valueof zero. Resistor 60 adjusts the impedance to produce a very low slopefor portion 533 of waveform 508, insuring effective blanking over theentire period of 533. Diode 61 clamps the low value of Waveform 508 toground providing a more effective drive of transistor 52.

Time 1012 is the delay between the beginning of a note andthe beginningof operation of the blanking circuits. This delay does not cause anydifficulty since the need for blanking always commences sometime afterthe beginning of a note in the type of musical or input signal underconsideration. Moreover, time delay 1012 is actually an advantage inmany instances where the note begins with a speaking tone or a beginningtransient with contains a high portion of aperiodic components,non-harmonic components or components having a rapid shift in frequency.In these instances, delay period 1012 gives the signal time to stabilizebefore blanking begins, which otherwise could result in the blankingcircuit following a false frequency, or otherwisemalfunctioning.

With respect to capacitor 58 in the blanking feedback network,it merelyfunctions to produce the margin time 534 afforded by the rounded portion535 of waveform 508.

For certain types of aural inputs to the signal processing unit 11, asfor example where the input signal is of the less complex variety, i.e.,is nearly sinusoidal in form, the refined system embodying the sawtoothgenerator, its blanking circuit, and squarer circuit may not beessential. In such instance the clipped and inverted output waveform 502obtaindat the collector of transistor 22 may be applied directly throughthe diode 45a to point 48 in the amplitude control circuit, whereuponthe cycle by cycle amplitude control is obtained in the same manner asabove-described. For this purpose a switch 125 is provided. When it isclosed to contact 125a, the aural input signal is processed through therefined circuitry including the sawtooth generator, its blankingcircuit, and squarer circuit. However, when the switch 125 is closed tocontact 125b, the output of transistor 22, waveform 502, is coupleddirectly to the diode 45. Obviously, if desired, a processing unit oflower performance characteristics could be, built, providing only thedirect coupling of waveform .502 to the diode 45a, omitting the sawtoothgenerator, its

blanking circuit, and the squarer circuit.

As previously indicated, it is one of the principal purposes ofthepresent invention to take a musical input of one character andconvertit to a musical output of another character, as for example convertingthe music of one instrument to that of a different instrument. Toaccomplish this objectivesuccessfully, it is essential that one be ableto select the envelope of the signal during the processing operation, tocorrespond with that musical fvalue' desired at the-output. Thisoperation is exemplified and embodied in the networks 16' and selectorswitch 62. When switch 62 is in the position 63, the output signal 511is not affected, and it follows the amplitude envelope of the inputsignal 501. If the input were a plucked string,

the output envelope would be the same, and is illustrated ,as envelope701 of FIG. 5. The switch position 64- produces an amplitude envelopefor output waveform 511 shown in envelope 702, which has a more gradualbeginning and is not as percussive as that of envelope 701. The gradualbeginning is caused by capacitor 67 requiring time .to charge throughdiode 68 and thus lowering level 530 at the beginning of a.,note. Switchposition 65 results in the envelope 703 having a flat top and soundingorganlike. The flat top is caused by diode 68 being returned to a fixedvoltage so that level 530 never exceeds that voltage value. Switchposition 66 produces the amplitude envelope 704, which is a combinationof envelopes 702 and 703. It is contemplated that when the aural inputof the processing unit is an expressive musical source, position 63would most likely be used in order to permit the expressiveness of thesource to be reflected in the output. One or another of the switchpositions 64, 6-5, and 66 would most likely be used to vary the musicalexpression of the output when the input is provided by an instrumenthaving a fixed amplitude envelope, such for example as a guitar.

A further feature of the present processing system is a minimum inputamplitude cut off circuit. Because the signal to noise ratio becomessufiiciently low to cause erratic operation of the sawtooth wavegenerator 26, 27, 30, it is desirable to provide a cut off circuitoperable when the input signal amplitude falls below a certain value.This result is accomplished by taking the squared input signal aswaveform 502 at the collector output of transistor 22, limiting it bymeans of diode 71 to a certain maximum value, passing this signalthrough diode 73, and integrating the resultant rectified relativelynegative value of the signal on capacitor 74. So long as the waveform502 obtains an amplitude value in excess of the limiting thresholdestablished by diode 71, the relatively negative value therebyestablished on capacitor 74 biases the transistor 47 below its out offvalue. However, when the amplitude value of waveform 502 falls below thelimiting threshold of diode 71, capacitor 74 moves positive, changingthe bias on the base of transistor 47 to a conducting value, therebyblanking the base of transistor 42 and rendering the amplitude controlcircuit inoperative and cutting off any output from the presentprocessing unit.

Operation of this threshold cut off circuit is explained in greaterdetail with reference to the Waveform chart of FIG. 6. Waveform 1101 isselected to represent an input waveform, and waveform 1102 is thecorresponding squared wave appearing as the output of transistor 22. Atthe time point 1106, the amplitude of waveform 1102 becomes too smallfor reliable operation of the succeeding stages. Waveform 1103 is thatappearing at the negative side of diode 71 resulting from the clippingaction of the diode by voltage 1107 at the positive side of diode 71.Voltage 1107 is selected so as to reduce the value of waveform 1103 tozero at time 1108-, shortly before time 1106. Waveform 1103 is passed bycapacitor 72 and rectified by diode 73, producing waveform 1104 at thecapacitor 74. Resistor 75 is selected to provide a positive quiescentvalue on the base of transistor 47, causing it to conduct and short outthe signal on the base of transistor 42, thus preventing the amplitudecontrol circuit from passing any signal. When the peak value of theinput signal 1101 is such as to provide a peak value for waveform 1102greater than the voltage value 1107, the voltage integrated on thecapacitor 74 is maintained at a point where transistor 47 is cut off,permitting the amplitude control circuit to onerate normally, aspreviously described. Accordingly, Waveform 1105 is the amplitudeenvelope of the output signal waveform 511, in which portion 111.0reflects operation of the present threshold cut off circuit, and portion1109, in dotted line, is the tail end of envelope 1105 which wouldcontain aperiodic signals if it were not eliminated by the presentthreshold cut off circuit.

Pursuant to the foregoing description there is obtained at the output oftransistor 44 a symmetrical square waveform signal 511, having afrequency equal to the fundamental frequency of the input signal 501 (or1101 in FIG. 6), and whose amplitude is a cycle by cycle function of theinput signal amplitude which of course may be modified When desired bythe envelope shaping networks 61. Waveform 511 is dropped in amplitudeacross resistor 81 foot pitch, the waveform 505 is frequency divider, asindicated at 96, and then processed in the the same manner as indicatedin the present circuit by utilizing the input signal 501 as theamplitude control therefor, as indicated at 97 in FIG. 2.

vided a system for processing an aural input of one tone color, toobtain an aural output of a different character, wherein the output isrelated in cycle by cycle correspondence to the frequency and amplitudeof the input. It is understood, of course, that the specific embodimentherein disclosed is presented only for purposes of illustrating theinvention, and that modifications and variations will be apparent tothose skilled in the art. Accordingly, such modifications and variationsas are embraced by the spirit and scope of the appended claims arecontemplated as being within the purview of the present invention.

and coupled through capacitor 82 as square waveform 511 to theappropriate tone color filter units 13.

Simultaneously, square waveform 511 is differentiated by capacitor 83and resistor 84 to provide waveform 512. The negative spikes of thiswaveform are passed by diode 85 to a sawtooth generator comprising theresistance capacitance network 86, 87, 8-8 and 89. Between negativespikes of waveform 512, capacitor 86 charges to a positive value, andthen is suddenly discharged by a negative spike from waveform 512, thusforming the sawtooth wave- 1 form 513. This waveform is then inverted byamplifier '91 to waveform 515, and again inverted and amplified byamplifier 92 to waveform 516. Waveform 516 is then passed to theappropriate tone color filter unit 13.

By appropriate filtering of square waveform 514 and sawtooth waveform516, a signal of desired tone color is obtained, as is well understoodin the art, and this resultant signal is then amplified at 14, andpassed to the output speaker to provide an aural output.

The circuit shown in FIG. 2 is the processing circuit for an 8 footpitch, or the reference footage having the same pitch as the inputsignal. To obtain the next higher octave, or 4 foot pitch, the 4 footcoupler switch 93 is closed connecting the square wave output channel 94and point 95 in the sawtooth output channel. On closing switch 93,waveforms 514 and 515 are added, and with waveform 515 chosen to have apeak amplitude twice that of waveform 514, the output of the sawtoothchannel is a sawtooth wave of double the frequency of waveforms 515 or514. This frequency doubling action is more fully explained in mycopending application, Ser. No. 474,892, filed July 26, 1965, andentitled Frequency Doubler and Coupler for Electronic Music GenerationSystems.

In addition, to obtain the next lower octave, or a 16 Where multipleprocessing units are used, with a sep- Thus, pursuant to the presentinvention there is pro- What is claimed is: 1. In a system forconverting an aural input signal of one tonal character to an auraloutput signal of a different tonal character:

(a) a processing unit comprising means for detecting a single fixedreference point in each cycle of the fundamental frequency of said inputsignal;

(b) said detecting means comprising means for generating a pulse for aselected direction of zero cfo'ssing for each cycle of the fundamentalfrequency of said input signal;

(c) said processing unit further including means responsive to saiddetecting means for generating a blanking signal extending from shortlyafter a given detecting means pulse to shortly before the nextsucceeding detecting means pulse to blank out extraneous detecting meanszero crossing pulses that may occur during a latter portion of a complexaural input tone; and

' (d) means responsive to said detecting means for generating one cycleof a square wave signal for each of said detected fixed reference pointsin cycle-forcycle correspondence with the cycles of said input signal.

2. In a system for converting an aural input signal of one tonalcharacter to an aural output signal of a different tonal character:

(a) a processing unit comprising means for detecting a single fixedreference point in each cycle of the fundamental frequency of said inputsignal, said detecting means comprising:

( 1) means for clipping the input signal to obtain the fundamentalfrequency thereof,

(2) means for differentiating the clipped signal to obtain pulsesdefinitive of zero crossings of the input signal, and

(3) means for rectifying the differentiated signal to obtain thosepulses definitive of only one selected direction of zero crossing; and

(b) means responsive to said detecting means for generating one cycle ofa symmertical waveform signal for each of said detected fixed referencepoints in cycle for cycle correspondence with the cycles of said inputsignal.

3. A system according to claim 2, said generating means comprising,

(1) a sawtooth waveform generator producing one cycle in response toeach of said rectified pulses, and

(2) a square waveform generator responsive to the sawtooth generator andproviding a linear and bilateral load therefore producing onesymmetrical square waveform cycle in response to each sawtooth waveformcycle.

4. In a system as set forth in claim 3, said unit further including asecond rectangular waveform generator responsive to said sawtoothwaveform generator and providing a linear and non-bilateral loadtherefor producing one non-symmetrical rectangular Waveform cycle inresponse to each sawtooth waveform cycle, means for coupling the outputof said rectangular waveform generator to the output of said rectifyingmeans during a portion of the cycle of said non-symmetrical rectangularwaveform to blank out extraneous pulses that may appear in th output ofsaid rectifying means between successive pairs of rectifying meansoutput pulses definitive of zero crossings of the fundamental frequencyof said input signal, as may occur during a latter portion of a complexaural input tone.

5. In a system as set forth in claim 3, said unit further including anamplitude control circuit comprising means for detecting the peak valueof a given half of each cycle of said input signal, and means forcombining said detected peak value with the corresponding cycle of saidsquare waveform to provide a second symmetrical square waveform in cycleby cycle synchronism with the first mentioned symmetrical squarewaveform and having an amplitude corresponding with the peak value ofthe corresponding input signal cycle.

6. In a system as set forth in claim 5, said unit further includingmeans for varying the amplitude envelope of a series of said secondsymmetrical square waveform cycles in accordance with a selectedfunction.

7. In a system as set forth in claim 5, said unit further includingmeans responsive to the amplitude of said clipped input signal forreducing to zero the amplitude of said second symmetrical squarewaveform when the amplitude of said clipped input signal falls below aselected value.

8. In a system as set forth in claim 7, said unit further including afirst output channel coupling said second symmetrical square waveform totone color filter means, a second output channel including means forconverting said second symmetrical square waveform to a correspondingsawtooth waveform and coupling it to tone color filter means, and meansfor converting the outputs of said tone color filter means to sound.

9. In a system for converting an aural input signal of one tonalcharacter to an aural output signal of a differenttonal character:

' (a) a processing unit comprising means for detecting a single fixedreference point in each cycle of the fundamental frequency of said inputsignal;

(b) said processing unit further including an amplitude control circuitcomprising means for detecting th peak value of a given half of eachcycle of said input signal;

(( means responsive to said detecting means for generating one cycle ofa symmetrical waveform signal for each of said detected fixed referencepoints in cycle for cycle correspondence with the cycles of said inputsignal; and

((1) means for combining said detected peak value with the correspondingcycle of said symmetrical Waveform to provide a second symmetricalwaveform in cycle-by-cycle synchronism with the first mentionedsymmetrical waveform and having an amplitude corresponding with the peakvalue of the corresponding signal cycle.

10. In a system as set forth in claim 9, said symmetrical waveformsignal generating means being a square waveform generating means, andsaid second symmetrical waveform being a square waveform.

11-.' In a system as set forth in claim 10, said detecting meanscomprising means for generating a pulse for a selected direction of zerocrossing for each cycle of the fundamental frequency of said inputsignal.

12. In a system as set forth in claim 9, said detecting means comprisingmeans for generating a pulse for a selected direction of zero crossingfor each cycle of the fundamental frequency of said input signal.

13. In a system as set forth in claim 9, said unit further includingmeans responsive to said detecting means for generating a blankingsignal extending from shortly after a given detecting means pulse toshortly before the next succeeding detecting means pulse to blank outextraneous detecting means zero crossing pulses as may occur during alatter portion of a complex aural input tone.

14. In a system as set forth in claim 13, said symmetrical waveformsignal generating means being a square waveform generating means, andsaid second symmetrical waveform being a square waveform.

15. A system for converting an aural input signal of one tonal characterto an aural output signal of a different tonal character, including:

(a) a processing unit comprising means for detecting the fundamentalfrequency of said input signal;

(b) means responsive to said detecting means for generating asymmetrical waveform signal in cycle-forcycle correspondence with thecycles of said input signal;

() amplitude means responsive to the amplitude of said input signal toadjust the amplitude of said symmetrical waveform signal; and

((1) means for selectively varying the harmonic content of saidsymmetrical waveform signal.

16. A system according to claim 15, including means for converting theresultant signal to sound.

17. A system according to claim 15, including means for varying theamplitude envelope of a series of cycle of said symmetrical Waveformsignal in accordance with a selected function.

18. In a system for converting an aural signal of one tonal character toan aural output of a different tonal character, means for determiningthe fundamental frequency of said signal, means for generating asymmetrical Waveform signal of said frequency, and means for varying theamplitude of each cycle of said symmetrical waveform signal inaccordance with the peak value of the corresponding cycle of said auralsignal.

19. In a system for converting an aural signal of one tonal character toan aural output of a different tonal character:

(a) means for determining the fundamental frequency of said signal;

(b) means for generating a symmetrical waveform signal of saidfrequency;

(c) means for varying the amplitude of each cycle of said symmetricalwaveform signal in accordance with the peak value of the correspondingcycle of said aural signal;

(d) means for selectively varying the harmonic content of saidsymmetrical waveform signal; and

(e) means for converting the resultant signal to sound.

20. In a system as set forth in claim 19, means for varying theamplitude envelope of a series of cycles of said symmetrical waveformsignal in accordance with a selected function.

21. In a system for converting an aural input signal of one tonalcharacter to an aural output signal of a different tonal character, aprocessing unit comprising means for converting an aural input signal toa signal substantially rectangular in waveform and having a frequencyequal to the fundamental frequency of said input signal, an amplitudecontrol circuit including means for detecting the peak value of a givenhalf of each cycle of said input signal, and means for combining saiddetected peak value with the corresponding cycle of said rectangularwaveform signal to provide a resultant signal in cycle by cyclesynchronism with the input signal and having an amplitude correspondingwith the peak value of the corresponding input signal cycle.

22. In a system for converting an aural signal of one tonal character toan aural output of a different tonal character, means for determiningthe fundamental frequency of said signal, means for determining the peakamplitude value of each aural signal cycle, and means responsive to thefirst two means for producing a signal having a frequency equal to thefundamental frequency of the aural signal and each cycle having anamplitude related to said peak amplitude of the corresponding cycle ofsaid aural signal.

23. A system for converting an aural input signal of one tonal characterto an aural output signal of a different tonal character, a processingunit comprising means for detecting the fundamental frequency of saidinput signal, means responsive to said detecting means for generating asymmetrical waveform signal in cycle for cycle correspondence with thecycles of said input signal, and amplitude means responsive to theamplitude of said input signal to adjust the amplitude of saidsymmetrical waveform signal.

24. In a system for converting an aural input signal of one tonalcharacter to an aural output signal of a different tonal character:

(a) a processing unit comprising means for converting an aural inputsignal to a signal substantially rectangular in waveform and having afrequency equal to the fundamental frequency of said input signal;

(b) an amplitude control circuit including means for detecting the peakvalue of a given half of each cycle of said input signal; and

(c) means for combining said detected peak value in synchronism witheach cycle of said rectangular waveform signal to provide a resultantsignal having an amplitude corresponding with the peak value of thecorresponding input signal cycle.

25. A system for converting an aural input signal of one tonal characterto an aural output signal of a different tonal character:

(a) a processing unit comprising means for detecting the fundamentalfrequency of said input signal;

(b) means responsive to said detecting means for generating asymmetrical waveform signal in cycle-forcycle correspondence with thecycles of said input signal;

(c) amplitude means responsive to the amplitude of said input signal toadjust the amplitude of said symmetrical waveform signal; and

(d) said processing unit further including means responsive to saiddetecting means for resetting said amplitude means extending for only aportion of a cycle of said symmetrical waveform signal.

26. A system for converting an aural input signal of one tonal characterto an aural output signal of a different tonal character:

(a) a processing unit comprising means for detecting the fundamentalfrequency of said input signal;

(b) said detecting means comprising low pass filter means for said inputsignal to obtain the fundamental frequency thereof;

(c) means responsive to said detecting means for generating asymmetrical waveform signal in cycle-forcycle correspondence with thecycles of said input signal;

(d) said generating means comprising a first sawtooth waveform generatorproducing one cycle in response to each cycle of the fundamentalfrequency;

(e) amplitude means responsive to the amplitude of said input signal toadjust the amplitude of said symmetrical waveform signal; and

(f) a second sawtooth waveform generator responsive to the symmetricalwaveform signal and producing one sawtooth waveform cycle in response toeach symmetrical waveform cycle.

27. A system for converting an aural input signal of one tonal characterto an aural output signal of a different tonal character:

(a) a processing unit comprising means for detecting the fundamentalfrequency of said input signal; (b) means responsive to said detectingmeans for generating a symmetrical waveform signal in cycle-forcyclecorrespondence with the cycles of said input signal; and

(c) amplitude means responsive to the amplitude of said input signal toadjust the amplitude of said symmetrical waveform signal, said amplitudemeans further comprising (1) means for detecting the peak value of agiven half of each cycle of said input signal, and

(2) means for combining said detected peak value with the correspondingcycle of said symmetrical waveform to provide a second symmetricalwaveform in cycle-by-cycle synchronism with the first mentionedsymmetrical Waveform and having an amplitude corresponding with the peakvalue of the corresponding input signal cycle.

28. A system as set forth in claim 27, including a first output channelcoupling said second symmetrical waveform to tone color filter means, asecond output channel including means for converting said secondsymmetrical waveform to a corresponding sawtooth waveform and couplingit to tone color filter means, and means for converting the outputs ofsaid tone color filter means to sound.

29. An amplitude control circuitcomprising means for detecting the peakvalue of a given half of an input signal, symmetrical waveform meansresponsive to said input signal to provide a first symmetrical waveformhaving a frequency octavely related to the fundamental frequency of saidinput signal, and means for combining said detected peak value with thecorresponding cycle of said first symmetrical waveform to provide asecond symmetrical waveform in cycle by cycle synchronism with saidfirst symmetrical waveform and having amplitude corresponding with thepeak value of the corresponding signal cycle.

30. A circuit as set forth in claim 29, wherein said first symmetricalwaveform signal generating means being a square waveform generatingmeans, and said second symmetrical waveform being a square waveform.

References Cited UNITED STATES PATENTS 3,094,666 6/1963 Smith 328-DONALD D. FORRER, Primary Examiner B. P. DAVIS, Assistant Examiner Us.01. X.R. 84-124, 1.19; 307 235 Patent No.

Inventor(s) Dated November 4, 1969 D. J. Campbell It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Col. 1, line 28, change "ampitude" to amplitude Col. 5, line 63, change"16" to 61 Col. 7, line 35, after "is" insert fed to a Col. 8, line 21,change "symmertical" to symmetrical Col. 9, line 63, change "cycle" tocycles SIGNED AND SEALED JUL? 1970 (SEAL) Attest:

Edward M. Fletcher, Jr. mun E i m. Amming 0mm Commissioner of PatentsFORM PO-105O (10-69)

